Modern vehicles utilize many actuators, controlling various aspects of vehicle operation. Many of these actuators control engine operation, such as throttle, cam phase, fuel injection, and spark timing. Other actuators control delivery of the engine torque to a vehicle's wheels, such as a torque converter or a transmission. Operation of these actuators must be coordinated to achieve acceptable vehicle performance. In particular it is desirable to control a vehicle to provide optimum fuel efficiency with acceptable NVH (noise, vibration, harshness) performance.
Fuel efficiency of many types of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermodynamic efficiency through the use of a smaller displacement when full torque is not required. The most common method of varying the displacement today is deactivating a group of cylinders substantially simultaneously. In this approach no fuel is delivered to the deactivated cylinders and their associated intake and exhaust valves are kept closed as long as the cylinders remain deactivated.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously and remain deactivated as long as the engine remains in the same variable displacement mode. Thus, the sequence of specific cylinders firings will always be exactly the same for each engine cycle during operation in a variable displacement mode (so long as the engine remains in the same displacement mode), whereas that is often not the case during skip fire operation. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed displacement modes.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns. One of the Applicants, Tula Technology has filed a number of patents describing various approaches to skip fire control.
Many skip fire controllers are arranged to provide a set of available firing patterns, sequences or firing fractions. In some circumstances the set of available firing patterns or fractions will vary as a function of various operating parameters such as engine load, engine speed and transmission gear. Typically the available firing patterns are selected, in part, based on their NVH characteristics. Transitions between firing fraction levels must be managed to avoid unacceptable NVH during the transition. In particular, changes in the firing fraction must be coordinated with other engine actuators to achieve smooth firing fraction transitions.
Many internal combustion engines incorporate a cam phaser to adjust a cam angle or phase relative to the crankshaft. Adjusting the cam phase varies the relative timing of the opening and closing of the intake and/or exhaust valves relative to top dead center (TDC) or some other crankshaft reference point. The cam phase impacts both the cylinder mass air charge (MAC) and the amount of residual exhaust gases left in the cylinder from the preceding cylinder working cycle.
Some engine valve trains utilize a single camshaft to actuate both intake and exhaust valves, while others utilize separate camshafts for the intake and exhaust valves. Still other engines have cylinders arranged in banks with single or dual camshafts dedicated to each bank. When a cam phaser is used in conjunction with a camshaft that actuates both intake and exhaust valves, then cam phase adjustments will affect both the intake and exhaust strokes. When dual camshafts that independently actuate intake and exhaust valves are used, then the intake and exhaust valve timings may be independently varied.
The cam phase can be set to provide optimum fuel efficiency (or other desired characteristics), however the optimal cam phase varies as a function of the engine speed and the cylinder load. Therefore, the fuel efficiency of an engine may generally be improved by varying the cam phase based on the engine operating conditions.
In addition to cam phase there are other actuators and control systems in modern vehicles that impact fuel efficiency and occupant comfort. One such system is control of the torque converter slip. The torque converter transfers motive power between the vehicle's engine and wheels. Torque converter slip indicates the difference in rotational velocity between the input, engine side, of the torque converter and the output, wheel side, of the torque converter. For fuel efficiency it is desirable to minimize or eliminate slip; however, insufficient slip will cause unacceptable NVH and compromise a vehicle's drivability.
There is need for control methods that coordinate changes in the firing fraction with adjustment of other vehicle actuators, such as cam phase and torque converter slip. The present application describes approaches for combining control of various vehicle actuators with skip fire operation to provide fuel efficient transitions between different firing patterns, sequences or firing fractions. In particular, control of cam phasing and torque converter slip are described, but the concepts presented herein are applicable to a broad range of vehicle actuators.